1. Field of the Invention
This invention relates to a medical image displaying apparatus for reproducing a medical image, which has been recorded during the recording of an X-ray and displaying it.
2. Description of the Prior Art
Systems have heretofore been used widely wherein image signals representing medical images are obtained with the X-ray CT, the MRI, or the like, and the medical images are reproduced as visible images from the image signals and displayed on image display devices, such as CRT display devices.
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. As a result, a medical radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the medical radiation image of the object as a visible image on a display device, such as a cathode ray tube (CRT).
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order for the desired image density to be obtained, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal (image signal) to be used in the reproduction of a medical radiation image as a visible image on an image display device, such as a CRT.
In the radiation image recording and reproducing systems described above, before an image is reproduced from an image signal and displayed on a CRT display device, or the like, gradation conversion processing is carried out in accordance with a look-up table (LUT), or the like, on the image signal such that a visible image having a desired gradation can be obtained. A visible image is then reproduced from the processed image signal and displayed.
However, a CRT display device, or the like, has characteristics which are very nonlinear, so that the visual brightness (image density) of a visible image displayed thereon is not proportional to the image signal fed thereinto. Therefore, even if the gradation conversion processing is carried out on an image signal and a visible image is reproduced from the processed image signal, a visible image having a desired gradation cannot be displayed on the CRT display device, or the like.
In order for the aforesaid problems to be eliminated, the nonlinear characteristics of an image display device may be found and taken into consideration when an image signal is converted in accordance with a LUT, or the like, so that a visible image having a desired gradation can be reproduced from the converted image signal and displayed on the image display device. With this technique, a visible image having a desired gradation can be displayed. However, a CRT display device, or the like, has the characteristics which are considerably nonlinear. Therefore, the problem occurs in that an image region, in which the image density changes continuously and smoothly, is displayed as an image having step-like changes in the image density, i.e. striped artifacts appear on the visible image. Such a visible image is not suitable for medical diagnoses.